The present invention relates to time ratio control systems, and more particularly to direct current time ratio control systems of the type utilizing a capacitor for periodically commutating a main thyristor.
Control systems for metering electric power from a direct current (d-c) source to a d-c load have long been known. In one prior art approach a plurality of resistances were inserted between a d-c load and a d-c source and the resistances selectively switched in or out of the circuit to vary the average voltage applied to the load. Due to I.sup.2 R losses in the resistors and the discrete changes in applied power, this approach, although simple, is not as efficient and smooth as is sometimes desired. With the availability of thyristors and more particularly the silicon controlled rectifier (SCR) a better approach, termed pulse control or time ratio control, has become feasible. As applied to systems for controlling power from a d-c source to a d-c load, time ratio control circuits are generally known as chopper circuits.
In time ratio controlled d-c power circuits, the load current conducting SCR is operated in the manner of a rapidly actuated switch thereby repetitively connecting and disconnecting the load to the source. By varying the average percentage of time in any one cycle that the SCR is conductive, the average power applied to the load may be varied correspondingly. In order to provide smooth operation, it is necessary to operate the SCR at a repetition rate such that the inherent characteristics of the load and any additional smoothing reactor will serve to smooth or integrate the pulses of electrical power.
The SCR is a three-terminal device having anode, cathode and gate terminals. When the SCR is forward biased, i.e., the anode terminal is at a positive potential with respect to the cathode terminal, a current signal applied to the gate terminal will cause the SCR to be gated into conduction and to exhibit a negligible anode to cathode resistance. Once gated or fired in this manner, the SCR can only be turned off by subsequently reducing the current through the device to zero and then applying a reverse bias from anode to cathode for a time period sufficient to allow the SCR to regain its forward voltage blocking ability. In practical applications the SCR can be turned off by means of a "commutation" circuit connected in parallel therewith. The combination of the commutation circuit and a load current carrying SCR is referred to as a chopper. A detailed description of SCR devices, chopper circuits and commutation circuits may be had by reference to the SCR Manual, Fifth Edition published in 1972 by the General Electric Company, Semiconductor Products Department, Syracuse, N.Y.
A typical chopper commutation circuit is a "ringing" circuit, i.e., the circuit contains inductive and capacitive components which develop an oscillating or ringing current. A chopper commutation circuit may include, for example, a capacitor, an inductor, several diodes and a commutating SCR. The chopping frequency is determined by the frequency at which the motor-current conducting main SCR and commutating SCR are fired, and the duty factor is determined by the percentage of a period between consecutive firings of the main SCR that has elapsed when the auxiliary SCR is fired. The capacitor is charged from the d-c power source and thereafter discharged in a manner which provides an auxiliary current path such that current through the main SCR is reduced below a conduction sustaining level and a reverse bias potential is effected across the main SCR whereby the main SCR is turned off or commutated. The auxiliary SCR itself is turned off by ringing action in the commutation circuit. In classifying chopper circuits according to commutating methods, two basic types of chopper circuits are known. A first type is termed a current turn-off chopper circuit and a second type is termed a voltage turn-off chopper circuit.
In a current turn-off chopper circuit the capacitor discharge path includes the load current carrying or main SCR which is to be commutated. This characteristic of the current turn-off chopper circuit requires that the load current carrying SCR be oversized in order to accommodate the commutation current in addition to the load current. To avoid the necessity of having commutating current pass through the load current carrying SCR, many systems utilize the voltage turn-off chopper circuit. In this latter circuit an auxiliary path bypassing the main SCR is provided for the commutating current and only a commutating voltage is applied to the main SCR. This commutating voltage is developed by reversing the voltage on the capacitor during the commutating interval. The voltage on the capacitor is essentially equal to source voltage so that the voltage reversal results in substantially twice the source voltage being impressed across the load connected to the chopper. For many loads repetitive application of twice source voltage during commutation may result in damage to the load.
It is an object of the present invention to provide an improved voltage turn-off chopper circuit in which load voltage magnitude is limited to source voltage magnitude.